1. Field of the Invention
The present invention relates to a copier, facsimile apparatus, printer or similar image forming apparatus and more particularly to a developing method using a two-ingredient type developer and an image forming apparatus using the same.
2. Description of the Background Art
Generally, an electrophotographic or similar image forming apparatus include either one of two different types of developing devices, i.e., one using a two-ingredient type developer consisting of toner and magnetic carrier and the other using a one-ingredient type developer, i.e., toner. A developing device using a two-ingredient type developer usually includes a rotatable sleeve or developer carrier accommodating a magnetic roller provided with a plurality of magnetic poles. Magnetic carrier grains on which toner grains are deposited are magnetically caused to deposit on the sleeve and conveyed by the sleeve to a developing position where the sleeve faces an image carrier. At the developing position, the developer, forming a magnet brush, develops a latent image formed on the image carrier for thereby producing a corresponding toner image.
The carrier grains and toner grains constituting the two-ingredient type developer are charged by being agitated together, so that the toner grains are stably charged and form a relatively stable toner image. However, a problem with this type of developer is that the carrier grains deteriorate due to repeated development while the toner content of the developer and therefore the toner and carrier mixture ratio varies due to consumption. To cope with the variation of the toner and carrier mixture ratio, it is necessary to replenish fresh toner grains to the developer by using a toner content control device.
A developing device using the one-ingredient type developer or toner causes the sleeve to convey only the toner deposited thereon to the developing position and is therefore free from the problems stated above. However, this kind of developer cannot be stably charged. Further, a force that retains the toner on the sleeve is generally weak, and the toner cannot be conveyed in a desirable condition. In light of this, Japanese Patent Publication No. 64-12386, for example, proposes to increase the surface roughness of the sleeve for thereby enhancing the conveyance of the toner and therefore image quality.
The magnetic carrier grains must be protected from the filming of the toner grains thereon, be provided with a uniform surface configuration, be protected from oxidation and decrease in sensitivity to humidity, and be prevented from scratching or wearing the image carrier. Further, the carrier grains must be configured to extend the life of the developer and to control chargeability and adjust the amount of charge. To meet these requirements, it is a common practice to coat the carrier grains with, e.g., suitable resin to thereby form rigid, strong coating layers.
For example, Japanese Patent Laid-Open Publication No. 58-108548 discloses magnetic carrier grains coated with particular resin. Japanese Patent Laid-Open Publication Nos. 54-155048, 57-40267, 58-108549 and 59-166968, Japanese Patent Publication Nos. 1-19584 and 3-628 and Japanese Patent Laid-Open Publication No. 6-202381 each teach magnetic carrier grains with various additives added to coating layers. Japanese Patent Laid-Open Publication No. 5-273789 proposes magnetic carrier grains with an additive deposited on their surfaces. Japanese Patent Laid-Open Publication No. 9-160304, teaches magnetic carrier grains each being covered with a coating layer containing conductive grains greater in size than the coating film. Japanese Patent Laid-Open Publication No. 8-6307 proposes to use a benzoguanamine-n-butylalcohol-formaldehyde copolymer as the major component of a carrier coating material.
Further, Japanese Patent Laid-Open Publication No. 2001-188388 discloses carrier grains covered with coating layers, which contain at least binder resin and grains each, and characterized in that the grain size D of the grains contained in the coating layers and the thickness h of the binder resin layer lie in the range of 1<D/h<5. In this condition, the grains of the coating layers protrude from the coating layers and can absorb, when the developer is charged by agitation, impactive contact occurring on the binder resin due to friction between the carrier grains and toner grains or between the carrier grains. This successfully prevents the toner grains from being spent on the carrier grains and prevents the binder resin where charge is expected to be generated from being shaved off, thereby causing the surface configuration of the carrier grains to vary little despite aging. In addition, the durability of the carrier grains is enhanced.
Hereinafter will be described problems. (1) through (3) of the conventional technologies to which the present invention addresses.
(1) A current trend with the developing device of the type using the two-ingredient type developer is toward a small carrier grain size for reducing brush marks and granularity and thereby enhancing image quality. However, the fluidity of the carrier tends to decrease with a decrease in carrier grain size, making it difficult for the developer to be deposited on the sleeve. Consequently, the amount of the developer deposited on the sleeve, i.e., conveyed by the sleeve via a doctor or metering member for a unit area tends to decrease or the deposition of carrier grains on the image carrier is likely occur. Particularly, the amount of deposition of the developer on the sleeve decreases with the elapse of time due to the wear of the sleeve, the deterioration of the developer, and the variation of frictional resistance ascribable to toner filming on the sleeve. Consequently, a decrease in carrier grain size results in a decrease in the amount of deposition of the developer on the sleeve that adversely effects image quality, making the amount of the developer to reach the developing position short.
(2) In the developing device of the type using the two-ingredient type developer, assume that the gap between the sleeve and the image carrier is reduced to enhance the developing ability and therefore image quality, which includes stable image density and reproducibility, as stated earlier. Then, the distance between the sleeve and the carrier grains present on the tips of brush chains, which form a magnet brush, is reduced, so that a magnetic force acting on the carrier grains is intensified to thereby cause a minimum of carrier grains to deposit on the image carrier. Reducing the gap for development is therefore effective to enhance image quality.
Now, to allow the two-ingredient type developer to be conveyed to the developing position in an adequate amount, the amount of the developer to deposit on the sleeve is controlled in accordance with the gap for development. More specifically, the lower limit of the amount of deposition is selected such that brush marks do not appear in an image. Also, the upper limit of the amount of deposition is selected such that the overflow of the developer, the locking of the sleeve, the adhesion of the developer to the sleeve and other troubles ascribable to the packing of the developer in the above gap do not occur. It is to be noted that because the packing mentioned above is more likely to occur as the gap becomes greater, it is necessary to lower the upper limit of the amount of deposition. More specifically, the smaller the gap, the narrower the adequate range of the amount of deposition available.
On the other hand, the amount of deposition of the developer on the sleeve involves irregularity due to the tolerance of a so-called doctor gap between the doctor and the sleeve, which is, in turn, ascribable to the dimensional accuracy or the mounting accuracy of the doctor or that of the sleeve. Also, as for aging, the amount of deposition tends to decrease due to, e.g., the wear of the surface of the sleeve, the deterioration of the developer, and the variation of frictional resistance ascribable to toner filming on the sleeve. Further, when the gap for development is reduced, the doctor gap should also be reduced in order to reduce the amount of deposition. This, however, makes stress exerted by the doctor on the developer heavy and is apt to accelerate the deterioration of the developer, further reducing the amount of deposition. Therefore, the prerequisite with the developing device with a narrow gap for development is that the deterioration of the developer ascribable to aging and therefore the variation of the amount of deposition be reduced.
(3) In the developing device using the two-ingredient type developer, high-quality images free from background contamination and toner scattering are achievable if the developer containing adequately charged toner grains is conveyed to the developing position so as to develop a latent image formed on the image carrier with the toner grains. To charge the toner grains to an adequate charge value, it is necessary to stably charge the toner and therefore to increase the shaft torque of the sleeve to a certain degree. However, an increase in the shaft torque of the sleeve directly translates into heavy stress to act on the developer, aggravating the toner spent condition on the carrier. This makes it difficult to adequately charge the toner grains despite aging and therefore adversely effects the resulting image. Particularly, when a document with a high image area ratio, i.e., a solid image is copied in a repeat copy mode, toner charging is not fast enough to meet the need with the result that the influence of the toner spent condition appears as a critical image defect.
The developer may be stably conveyed to the developing position if much developer is held at a position downstream of the doctor. For this purpose, the doctor may be formed of a magnetic material or the flux density of the pole of a magnet roller facing the doctor may be increased. This kind of scheme, however, makes the stress acting on the developer excessively heavy, further aggravating the toner spent condition.
Further, in parallel with the trend toward oil-less fixation, wax-containing toner is replacing the conventional fixing oil. However, the problem with wax-containing toner is that wax leaks from the toner and aggravates the toner spent condition.
(4) The smaller carrier grain size meeting the need for higher image quality, as stated earlier, brings about a problem that magnetization is reduced to such a degree that the carrier grains deposit on the image carrier. Also, to meet the increasing demand for a small-size image forming apparatus, the diameter of a photoconductive drum, which is a specific form of the image carrier, and that of the sleeve are decreasing. However, a decrease in the diameter of the drum or that of the sleeve causes magnetic restraint acting on the carrier grains, which are present on the tips of the brush chains downstream of the developing position, to decrease, aggravating the carrier deposition on the drum. As a result, the drum, a cleaning blade and an intermediate image transfer body are more rapidly deteriorated while an image is locally omitted due to the carrier grains deposited on the drum.
To reduce the carrier deposition on the drum, the magnetic force of, among the poles of the magnet roller disposed in the sleeve, a main pole facing the drum and that of a pole downstream of the main pole may be intensified. This kind of-scheme increases magnetic restraint on the carrier grains being conveyed away from the main pole toward the downstream pole to thereby obstruct the separation of the carrier grains from the magnet brush.
Alternatively, the resistance of the carrier grains may be lowered to allow counter charge left on the carrier grains after the development of a solid image to be easily dissipated, thereby reducing the deposition of the carrier grains on the edge portions of an image ascribable to the counter charge. However, a decrease in the resistance of the carrier grains is likely to cause the charge to easily leak, so that defective images occur when use is made of an AC bias for development.
Further, the gap for development may be reduced for the same purpose. This, however, brings about the problems stated earlier. More specifically, although the initial amount of deposition of the developer on the sleeve may be made relatively great in consideration of a decrease to occur due to the deterioration of the developer and that of the sleeve surface, this further intensifies the packing in the developing position due to the variation of the amount of deposition.
None of the schemes described above can reduce the carrier deposition on the drum to the allowable level alone in the condition wherein the carrier grain size or the diameter of the drum or that of the sleeve is reduced.
Technologies relating to the present invention are also disclosed in, e.g., Japanese patent Laid-Open Publication Nos. 5-19632, 5-66661, 11-327305, 2000-10336, 2000-47489, 2000-155462, 2000-250308, 2001-5293, 2001-188388 and 2002-62737.